Conventionally, it has been known to use a clad material composed of a bronze alloy, such as high tin bronze or phosphor bronze, having a high impact strength and a low-carbon steel having a high ductility, as a member used for a clutch of an automatic transmission for automobiles, such as an end plate of one-way clutch (see paragraph [0011] of JP-A-6-337026). The clad material is manufactured by scattering a bronze powder on a steel plate and repeating sintering and dry-rolling, in which processes a sintering temperature and rolling reduction are selected paying attention to grain growth of the steel plate at the sintering temperature and to densification of the bronze alloy, as stated in paragraph [0018] of JP-A-2003-269456. When the grains of the steel become gross or when the bronze alloy layer has a high porosity, working stress will be concentrated on grain boundaries of the steel or pores of the bronze alloy layer, which may become starting points of cracks at the time of working such as deep drawing or the like. Dry-rolling is adopted for the manufacturing process, because there is caused a problem, when wet-rolling is adopted in which a rolling oil is supplied to surfaces of rolling rolls, that the rolling oil enters through capillary phenomenon into pores deep in a sintered bronze alloy layer and in a subsequent step the entering oil vaporizes in the sintered layer during heat treatment for sintering to form further voids.
For the clad material composed of the bronze alloy having a high impact strength and a low-carbon steel having a high ductility, hardness of the bronze alloy becomes greater than that of the low-carbon steel in rolling. Thus, a ratio of reduction of the low-carbon steel in respect to total reduction of the clad material is increased, and so the bronze alloy is hard to be reduced in a case where a bronze alloy having a high strength is used, or a steel having a low strength is used. When a ratio of reduction is increased in excess in the dry-rolling, it will cause a problem that seizure is generated between surfaces of rolling rolls and surfaces of the clad material. Therefore, in order to adequately attain densification of the bronze alloy in the clad material, dry-rolling is performed in several stages so that dry-rolling and sintering are repeated, which has a disadvantage of bad production efficiency.